System for measuring vibrational energy



w i (we ummy June 20, 1939. M. s. MEAD, JR 2,163,404

SYSTEM FOR MEASURING VIBRATIONAL ENERGY Original Filed Dec. 13, 1934@ill Inventor:

i5 ttorney.

Patented June 20, 1939 PATENT OFFICE SYSTEM FOR MEASURING VIBRATIONALENERGY Milton S. Mead, Jr., Schenectady, N. Y., assignor to GeneralElectric Company, a corporation of New York Original applicationDecember 13, 1934, Serial No. 757,311, now Patent No. 2,082,646, datedJune 11, 1937.

Divided and this application September 27, 1935, Serial No. 42,416

4 Claims.

This application is a division of my copending application, Serial No.757,311, filed December 13, 1934, Patent No. 2,082,646 dated June 11,1937, and assigned to the same assignee as the present application.

My invention relates to vibration-responsive devices.

One of the objects of my invention is to provide a sensitivevibration-indicating instrument with a progressively varying scaledistribution, particularly one having a scale expanded at the lower end,such as a logarithmic scale.

Another object of my invention is to produce vibration-responsiveapparatus which may be adjusted to respond either to vibration velocityor to amplitude of vibration.

Other and further objects and advantages will become apparent as thedescription proceeds.

In order to measure small alternating-current voltages, various types ofvacuum tube amplifying circuits have been devised. When recordingapparatus or switchboard-indicating meters are to be operated inresponse to minute alternatingcurrent voltages, it is desirable toprovide an am- 25 plifier which will produce a strong direct currentvarying in response to variations in the alternating voltage or currentto be measured. In certain applications, it becomes desirable also thatthe device should have a tapered response. For instance, in apparatusfor measuring vibration which may vary in strength over very wideranges, the apparatus should be highly sensitive to Weak vibrations andrelatively less sensitive to strong vibrations in order that the entirerange of vibrations may be measured and very strong vibrations will notinjure the apparatus. Such apparatus will have a scale expanded at thelower ends. It is also desirable in certain types of apparatus toprovide a voltmeter which may be adjusted to have an inverse frequencycharacteristic. For example, in vibration-measuring apparatus voltagesare obtained which are proportional to vibration velocity or the productof the amplitude of vibration and frequency. In order to obtain readingsof amplitude directly, one may employ a voltmeter having an inversefrequency characteristic. My invention has for its object the provisionof a voltmeter satisfying these requirements.

In carrying out my invention in its preferred form I provide a pair ofdischarge tubes of the grid controlled type arranged in the well knownpush-pull connection. Means are provided for reducing the amplificationof the discharge tubes as the input voltage to be measured increases instrength. In this manner, an open scale is obtained in the lower portionof the scale range of the voltmeter.

The features of my invention which I believe to be novel and patentablewill be pointed out in the claims appended hereto. A betterunderstanding of my invention may be obtained from the followingdescription taken in connection with the accompanying drawing, in whichFig. 1 represents schematically a vibration-measuring apparatusemploying a tapered-response discharge-tube voltmeter in accordance withmy invention, and Fig. 2 is the circuit diagram of a modified form ofvoltmeter.

Referring now more in detail to the drawing in which like referencecharacters are used to designate like parts throughout, in Fig. 1, Ihave illustrated my vibration-measuring system utilizing the taperedresponse thermionic voltmeter. It will be understood, however, that thevoltmeter is not limited to this application but is useful indepently innumerous other applications where voltage must be measured.

The voltmeter comprises a pair of vacuum tubes II and :2 so that thepush-pull connection may be employed. I prefer to use the push-pullconnection for the sake of greater output and also to avoid undesirableeffects arising from feeding back A. C. voltage from plate to grid, butit will be understood that my invention may also be carried out withonly a single tube in the ordinary connection.

The discharge tubes II and I2 comprise anodes l3, cathodes l4, andcontrol grids Hi. The cathodes l4 may be either of the filamentary orindirectly heated type. To avoid confusion in the drawing, the currentsource for heating the cathodes l4 has not been shown since theconstruction and operation of such tubes are well known in the art.

In order to increase the power and improve the operating characteristicsof the apparatus, screen grids Hi may be employed. Anode circuits areformed between the anodes I3 and the cathodes M and are energized by asource of direct current IT. The negative side of the current source ll,represented as being at zero potential is connected to the cathodes I4.The positive side of the current source t'l is connected to a midtap l8of a reactor IS, the outer terminals of which are connected to theanodes l3. If the screen grids l6 are employed, they are connected by acommon lead to an intermediate potential tap 20 of the current sourcel1. Voltage (21, to be measured, is supplied in the grid circuits as aninput voltage.

For the push-pull connection, the input voltage is divided by means of aresistor 2| having its outer terminals connected to the grids 5 of tubesII and 2, respectively, and having its midtap connected to the commonterminal of the oathodes |4 through a grid-circuit resistor 22. In thisway, half the input voltage e1 is supplied in the grid circuit of eachtube as is well known in connection with push-pull circuits. In the caseof a simple single-tube circuit, of course, the entire voltage would beconnected in the one grid cir- Quit. Q

A grid-biasing condenser 23 is connected between anode 3 of tube II andthe terminal 24 on the input side of the grid-circuit resistor 22. Arectifier is connected across the grid-circuit resistor 22 with itspositive side connected to the input terminal 24. If desired, therectifier may comprise an anode 25 placed within the same envelope 26 asthe other elements of the tube so as to form, in co-operation with thecathode l4, a diode rectifier.

Included in the anode circuit is a current-responsive device 21 whichmay be of any desired type responsive to direct current. It may consist,for example, as shown, of a recording instrument having a movablecurrent-conducting coil 28 carrying a pen arm 29, operating with amoving chart 30 to produce a record curve. If desired, a scale 3| mayalso be provided to permit more easily reading the instantaneousindications as the record is being produced. The movable element 28 ofthe recording instrument 21 is provided with a biasing spring 32arranged to bias the instrument to the full scale position shown asbeing at the right-hand end of scale 3|, and the connections to thecurrent-conducting winding 28 are such that, with maximum direct-currentflowing therein, the pen arm 29 is deflected down scale to the zeroscale position.

In the application of the voltmeter illustrated in the drawing, theinput voltage 61 is derived from the vibrating coil 33 of avibration-measuring device 34, which may be of any desired type.Preferably, a step-up transformer 2 I is inposed between the coil 33 andthe thermionic voltmeter. In the particular application illustrated, Ihave shown a vibration-measuring unit which is particularly adapted formeasuring vibration of objects upon which the vibrationmeasuring unit 34may be rested or to which it may be fastened, such for example, as aturbine bearing 35. The vibration unit 34 consists of a base 36 incontact with the object, the vibration of which is to be measured, andan element 31 of relatively great mass including a permanent magnet 38with a cylindrical casing 39, a circular end plate 40, and an annularpiece 4| providing a magnetic return for the permanent magnet 38. Thereis also provided a rigid bracket 42 which serves to form a rigidconnection through a rod 43 and a coil-supporting member 44 between thebase 36 and the vibrating current-conducting coil 33. The connection isrigid so far as horizontal axial motion of the coil 33 is concerned. Theelement of relatively great mass 31 is supported upon the base 36 bymeans of springs 45 which permit the element 31 to remain relativelystationary as the base 36 and the bearing 35 are vibrated.

Accordingly, relative motion takes place between the conducting coil 33and the field of the permanent magnet 38, and the voltages induced inthe coil are proportional to its vibration velocity. As the motion ofthe coil 33 is oscillatory, an

alternating voltage will be induced therein. The instantaneous value ofthe voltage is proportional to the instantaneous velocity of the coil33. Accordingly, the maximum, effective, and average values of inducedvoltage will be proportional to the maximum, effective, and averagevalues of vibration velocity. By means of my vacuum tube amplifyingcircuit, an indication of vibration velocity may be obtained in thedirect-current instrument 21 even though the vibration velocity is solow as to produce only weak alternating voltages.

It is apparent that, for a given amplitude of vibration, the vibrationvelocity will increase with the frequency of vibration so that thevibration velocity and the induced voltage are proportional to theproduct of amplitude of vibration and frequency. In some cases, anindication of amplitude of vibration rather than vibration velocity isdesired, and, in such cases, I arrange the vacuum tube voltmeter in sucha manner that an inverse frequency response is obtained. For thispurpose, I provide a condenser 46 and switches 41 for connecting thecondenser 46 in parallel with the reactor I9. When inverse frequencyresponse is desired, the inductive reactance of the reactor I9 is madehigh with respect to the capacitive reactance of the condenser 46.

The operation of the vacuum tube voltmeter is as follows:

When the voltage e1 across the resistor 2| is zero, there is no gridbias, the anode currents in the vacuum tubes and I2 are a maximum and amaximum direct-current flows in the movable winding 28, deflecting thearm 29 to the zero indicating position. Since the anode currents flow inthe opposite direction through the reactor |9, the magnetizing effect ofthe direct currents will cancel and no saturation effects are producedin the reactor l9. When an alternating voltage appears across theresistor 2|, this voltage will be amplified by the tubes II and I2 andan increased alternating-current voltage will be produced between theanode l3 and the cathode l4 of the tube owing to the presence of thereactor I9 in the anode circuit. The alternating voltage charges thecondenser 23 to its peak value through the diode rectifier 25, thearrangement being such that the left-hand plate of the condenser 23 isnegative. The resistance of the resistor 22 is made so high that thenegative charge cannot readily leak ofi. Accordingly, a negative-biasingpotential will be impressed on the grids l5 of tubes and I2.

In consequence, as the alternating voltage e1 increases in effectivevalue, the potentials of the grids I5 are depressed and theamplification of the tubes is decreased. The direct current through thecurrent-conducting winding 28 of the instrument 2'! will also decreaseas the voltage 61 increases and, due to the decrease in theamplification factor, successive increments in effective value ofalternating voltage at 61 will produce smaller effects on the anodecircuit so that a tapered response is obtained. In this manner, theapparatus responds with great sensitivity to very minute vibrations and,as the strength of vibration increases, the effect is decreased so thata very wide range of vibration may be recorded on the chart 39. If theconstants are properly chosen, a logarithmic response may be obtainedfrom the instrument 21.

If it is desired to measure amplitude of vibration rather than vibrationvelocity, the switches 41 are closed. Since the impedance of the reactorI 9 was made very high with respect to that of the condenser 46, thecombined reactance will vary substantially as that of a condenser andwill vary inversely with frequency. As the amplification of a screengrid discharge tube is proportional to the impedance in the anodecircuit. variation in grid bias and the response obtained areproportional to the quotient of vibration velocity and frequency orproportional to the amplitude of vibration.

For the sake of compactness, I prefer to use pentode tubes combining theanodes, cathodes, and control grids of the usual discharge tube and theelectrodes of the usual diode rectifier in a single envelope 26.However, it will be understood that, if desired, a separate rectifier,such as the diode rectifier 48, and either triode or screen-grid tubes49 in separate envelopes may be employed as illustrated in Fig. 2.

In accordance with the provisions of the patent statutes, I havedescribed the principle of operation of my invention together with theapparatus which I now consider to represent the best embodiment thereof,but I desire to have it understood that the apparatus shown is onlyillustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A device for indicating vibration amplitude of a vibrating body, saiddevice comprising in combination, a magnetic field structure of suchmass as to remain relatively stationary, a currentconductingvoltage-generating coil in inductive relation with said field structure,means for causing said coil to follow the vibration of the vibratingbody, and connected in responsive relation to said generating coil atapered response alternating-current voltmeter, the sensitivity of whichvaries inversely with input voltage and frequency.

2. Vibration amplitude responsive apparatus comprising in combination, amagnetic field structure and a current-conducting voltage-generatingcoil in inductive relation, one being so supported and of such mass asto remain relatively stationary when subjected to vibrations underinvestigation and the other having means for causing it to follow thevibration under investigation, a current-responsive device in responsiverelationship to said coil, and means for causing the relationshipbetween the currents in said current-responsive device and in said coilto vary inversely as the frequency of the voltage generated in saidcoil.

3. A device for indicating vibration amplitude of a vibrating body, saiddevice comprising in combination, a magnetic field structure of suchmass as to remain relatively stationary, a current-conductingvoltage-generating coil in inductive relation with said field structure,means for causing said coil to follow the vibration of the vibratingbody, and connected in responsive relation to said generating coil atapered response voltmeter means, the sensitivity of which variesinversely with input voltage and frequency.

4. Vibration amplitude and velocity responsive apparatus comprising incombination, a magnetic field structure and a current-conductingvoltagegenerating coil in inductive relation, one being so supported andof such mass as to remain relatively stationary when subjected tovibrations under investigation and the other having means MILTON S.M'EAD, JR.

